The CR Versus PS Debate: A Throwback Throw-Down in Total Knee Arthroplasty

Commentary In the context of contemporary firestorm podium debates in total knee arthroplasty (TKA), such as robotics versus manual techniques, press-fit versus cemented fixation, and kinematic versus mechanical alignment, the article by Young-Hoo Kim et al. is a throwback throw-down of sorts. The debate between posterior-stabilized (PS) and cruciate-retaining (CR) designs began in the 1970s, shortly after the inception of TKA. The goals of TKA are to alleviate pain and obtain a stable knee with improved range of motion and function. There has been an ongoing controversy over whether to retain or sacrifice the posterior cruciate ligament (PCL) in TKA. Modern TKA evolved from the Total Condylar Prosthesis (TCP), comprising symmetric femoral condyles articulating with a congruent tibial base plate in which the cruciate ligaments were excised1. Paradoxical anterior translation of the femur on the tibia resulted in decreased range of motion in flexion. Post-cam mechanisms of subsequent PS designs have been refined to improve femoral rollback and increase knee flexion. The theoretical basis of CR implants was to retain natural knee kinematics including femoral rollback, rotational stability, and mechanoreceptors within the ligament that confer proprioception. In contrast to PS articulations, CR implants have a round-on-flat articulation and rely on the PCL to prevent posterior translation of the tibia with respect to the femur. Despite biomechanical differences and an abundance of investigations comparing PS and CR designs, neither has manifested clinical superiority. In 2005, a methodologically rigorous systematic review and meta-analysis from the Cochrane Library—internationally recognized as representing the highest standard in evidence-based analyses—attempted to answer the question of whether to retain or sacrifice the PCL during TKA2. The answer remained unclear. An updated Cochrane review in 2013 also revealed no clinically relevant differences3. National joint registry data also show equivalent results4. The PS versus CR controversy was eventually supplanted on the podium by newer, flashier disputes over robotics, alignment, and fixation. Ostensibly, the PS versus CR debate had been laid to rest. Kim et al. present fresh methodology worthy of resurrecting the age-old debate. Their study—a randomized clinical trial comparing functional outcomes and implant survival in patients undergoing simultaneous, bilateral TKA with a CR implant randomized to one knee and a PS implant in the other knee—achieved what other studies have not: with a single-surgeon design and using consistent surgical technique and perioperative care, and with contemporary PS and CR designs of the same manufacturer implanted within the same patient, the authors have controlled for more variables than most in vivo arthroplasty studies. Furthermore, the study has nearly 2 decades of follow-up data, including radiographic and functional outcome results. A relatively small number of patients were lost to follow-up, and most had complete radiographic and functional outcome data. There were no differences in pain, satisfaction, implant survival, or function as measured by the Knee Society Score (KSS), Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) score, and the University of California Los Angeles (UCLA) activity score, and no differences in radiographic results. There were limitations to the study: the KSS, WOMAC, and UCLA activity scores are not designed for a patient to differentiate function pertaining to a specific knee. Carryover effects from one TKA could impact the results gathered from the other. Furthermore, the study population was homogenous demographically, with predominantly female Korean patients, all ≤65 years of age, with exclusively varus deformity; the results may not be generalizable to other populations. However, the study is a formidable achievement in terms of variables controlled, follow-up duration, and proportion of complete data. In summary, the findings by Kim et al. underscore what we have known about the PS versus CR controversy: that there is clinical equipoise. The beauty of our field is its perpetual state of evolution and innovation. New technology has since entered the playing field to address ongoing patient dissatisfaction associated with conventional (PS and CR) designs. Ultra-congruent (UC) and medial-pivot (MP) articulations were designed to recreate physiologic kinematics5. Their impact on patient satisfaction, function, and survival is, at present, undefined. At this juncture, we are well equipped to move past the PS-CR debate, don our boxing gloves, and enter the proverbial ring to settle the next match: UC or MP versus conventional bearings, anyone?

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